The present invention is generally related to integrated circuits including those provided logic functions, and more particularly to integrated circuits operating off of more than one voltage supply and having different voltage drops.
Integrated circuits (ICs) are widely used in today""s electronic equipment and provide the control functions thereof. These ICs can be fabricated according to numerous different semiconductor technologies including CMOS, Bi-polar, BiCMOS, ECL just to name a few. Circuits fabricated of these different semiconductor technologies are adapted to operate off of voltage sources having different nominal voltage requirements. For instance, CMOS devices typically operate off a +5 volt supply, with one pin being tied to ground. Bi-CMOS devices, however, are typically adapted to operate off of a +3.3 volt supply.
In electronic circuits including multiple control circuits adapted to operate off of different power supply voltages, it is conventional to either provide two different voltage sources, or, to provide a voltage source adapted to power one logic circuit, this voltage source being fed to a resistive divide circuit to level shift the voltage down to the nominal operating voltage for the second integrated circuit, as shown in FIG. 1. While these conventional techniques are adequate, they suffer in that the resistive divide network shunts a significant amount of current to ground, which is a power loss. Moreover, each of the different logic circuits, shown at 12 and 14, individually conduct the normal operating currents which are then provided to ground. In this shown example, the 5V logic circuit draws 25 milliamps from the 5 volt supply, the 3.3 V logic circuit draws 40 milliamps, and the resistive divide network shunts 4 milliamps to ground, a total draw of 69 milliamps. Sometimes, the resistive divide network 16 can include a Zener diode in combination with a resistor. However, this combination also undesirably shunts a significant amount of current to ground.
Other equivalents to the resistive divide network include level shifter circuits which can shift voltages up or down. However, these voltage converter circuits shunt a significant amount of current to ground, or, consume a large amount of power themselves.
There is desired an improved multiple logic circuit design having a single voltage supply which incorporates multiple control circuits operating off of different rail-to-rail voltages, which circuit does not shunt a significant amount of current to ground, and further, has increased efficiency and use of current provided by the single voltage supply.
The present invention achieves technical advantages as a stacked logic circuit whereby two or more logic circuits are arranged in series, and configured between a single voltage source and ground. A novel shunt circuit is provided in parallel with one logic circuit to shunt additional needed current from the voltage source to the second logic circuit via a shunt node, and without shunting a significant amount of current to ground. The second logic circuit being coupled in series with the first logic circuit thereby conducts the first logic circuits current, providing for a recycling of the current. The shunt circuit has a very low impedance such that large swings of current drawn from the voltage source does not significantly effect the voltage at the node between the logic circuits. A second shunt, such as Zener diode, is connected to the shunt node and is in parallel with the other logic circuit to sink current from the first logic circuit that is not conducted by the second logic circuit. Thus, these two shunt circuits provide load balancing for providing extra current to the second logic circuit, or drawing extra current circuit from the first logic circuit, depending on the current load balance.